Suppression of parasitic oscillations in high-frequency devices



J. w. MCRAE 2,422,695

SUPPRESSION OF PARASITIC OSCILLATIONS IN HIGH FREQUENCY DEVICES June24-, 1947.

2 Sheets-Sheet 1 Filed May 7, 1943 FIG.

INVENTOR J. W. MC RAE ATTORNEY Patented June 24, 194-7 UNETED STATESorrica SUPPRES'SIUN OF PARASITIC OSCILLATIONS IN MGR-FREQUENCY DEVICESJames W. McRae, Arlington, Va, assignor to Bel-l Telephone Laboratories,Incorporated, New York, N. Y, a corporation of New York 7 ApplicationMay '7, 1943, Serial No. 485,968

19 Claims. 1

This invention relates to high frequency electronic devices andparticularly those comprising space resonators. It relates particularlyto the technique of carrying electrical conductors through the spacewithin such a resonatorv for various purposes as for instance theplacing of direct current potentials upon the electrodes of an electrontube located within the resonator.

An object of the invention is to provide such conductors through thespace of the resonator without giving rise to unwanted electricaloscillation energy.

Another object is to provide such conductors without allowing therebyexcessive leakage or unwanted radiation of the high frequency energywith which the resonator is energized for a desired purpose.

In high frequency electronic devices a common method of excitingresonant cavities or space resonators for the purpose of producing highfrequency electrical energy s to pass an electron tube through theenclosed space through apertures in the shell of the cavity or resonatorso that an electron stream may be projected to interact with theenclosed high frequency electric field. Sometimes the resonator isincluded Within the evacuated envelope of the electron tube and theelectron stream is directed through apertures in the resonator.

In the excitation of a resonator by any such means it is common practiceto provide electrodes along the path of the electron stream Within theresonator and bias these electrodes suitably to control the electrons invarious ways. The control may be to accelerate, decelerate, focus, deflect or otherwise act upon the electron stream. The leads for chargingor biasing these electrodes must ordinarily pass through the highfrequency field of the resonator. These leads have electrical dimensionsof themselves and in combination with other elements of the device theyfrequently act to set up undesired electrical oscillations which absorbelectron energy and may cause damage in their dissipation.

According to this invention these unwanted oscillations are prevented byextending the circuit which tends to support them outside of theresonator and inserting there a damping resistance. At the same time thedissipation through this circuit of useful energy at the operatingfrequency is prevented by adjustment of the length of the circuit Withinthe resonator and the effective termination of it for the operatingfrequency at the boundary of the resonator.

The invention is more completely set forth in 2 the followingdescription and the associated drawings, of which:

Fig. 1 shows a high frequency oscillator utilizing the invention in oneform;

Fig. 2 is a section through a portion of the shell of the resonator ofFig. 1 to show a, form of construction;

Fig. 3 is a schematic circuit diagram used in explaining the inventionas embodied in Fig. 1;

Fig. 4 illustrates an application of the invention to an oscillatorcircuit which is a slight modification of Fig. 1;

Fig. 5 illustrates an alternative to the structure of Fig. 1;

Fig. 6 is a section through a portion of the shell of the resonator ofFig. 5 to show a form of construction;

Fig. '7 shows an application of the invention to an oscillator circuitusing a tube with one less electrode than the tube of Fig. 1;

Fig. 8 is a schematic circuit diagram used in explaining the inventionas embodied in Fig. '7.

Referring now to Fig. 1. This shows as an example a high frequencyoscillator circuit incorporating the invention' In the operation of thiscircuit the resonator l is energized at its resonant frequency by thethree-gap electron tube 2 utilizing the electron velocity variationmethod in the following manner. The electrons in passing from thecathode 4 to the collector It are exposed to the high frequency electricfield within the resonator as they .pass through each of the three gaps(between electrodes 6 and l, l and 8, and 8 and 9). In the first gap(between electrodes 8 and l) the electron velocities are varied inaccord with the impressed high frequency electric field. While passingthrough electrode 1 the electrons, because of the velocity variations,become partially grouped so that the electron stream acquires a degreeof electron density variation. In the second gap (between electrodes 1and 8) the density varied stream delivers energy to the high frequencyfield and at the same time additional velocity variations are impressedupon the electrons. In passing through electrode 8 further electrongrouping takes place so that the electron stream acquires a higherdegree of density variation. The electron stream then deliversadditional energy to the high frequency field in the third gap (betweenelectrodes 8 and 9) Devices operating on the same general principle mayhave a number of gaps greater than three or only two. The particularmethod of operation whereby high frequency energy is derived from theelectron stream is not important to an understanding of the inventionand therefore a more detailed discussion of it is not necessary here.The invention as embodied in Fig. 1 has to do with the matter ofelectrical connections to electrodes such as 1 and 3 within theresonator and incidental electrical effect arising from such connectionswhich may interfere with the proper operation of the device.

Fig. l is schematic in nature. It shows an axial section of theresonator I and the electron tube 2 which extends through the resonatoralong its axis and of which the electrodes 6, I, 8 and 9 and theirsupporting rings II, I2, l3 and I4 form part of the conducting shell ofthe resonator.

velope I5 may be of glass or other suitable insu.

lating material.

The electron emitting cathode 4 is indirectly heated by the heater .3from the energy source 3 I An alternating current source may besubstituted for the battery 31 indicated. The electron accelerator 5 andthe other electrodes of the tube are maintained at suitable potentialsthrough connections from source 32. A rectifier power supply system orother type of direct current power source may be substituted for thebattery 32. Under the influence f the positively charged electrodes 5,6, I, 8, 9 and It! a stream of electrons is caused to flow from thecathode along the axis of the tube (as indicated by the broken line) tothe collector 50 thereby producing electrical energy in the resonator Iat its resonant frequency as described briefly above. For the purpose offocusing the electron stream a unidirectional axial magnetic field maybe impressed in the vicinity of the gap between electrodes and 9 in aknown manner. Means for such is not shown in order to simplif thedrawing. The high frequency energy produced in the resonator may betransmitted to a load circuit through the coaxial line. I1, I8.

The electrodes 1 and 8 are maintained at the desired potential byconnections to the shell I6 (and thence to the source 32) through theleads I9, 20, 2!, 22, the coils 2.7, 28 and the resistors 29, 30 and thelead 23. The leads 2I and 22 pass through the insulating bushings 25 and.26v in the metallic member .24. which is connected to the shell I6 tobecome effectively a part of it and reduce appropriately the length ofthe leads I9 and within the resonator space. The relation of member2Ilto the cavity space is indicated in Fig. 2 which shows a sectionthrough member 24 and a portion of the shell It.

The leads I9, 20, 2|, 22 and 23, the members 24, 25 and 25, the coils 21and 28 and the resistors 29 and 39, are involved in this embodiment ofthe present invention. Apart from the inven tion, leads such as I9 and20 could connect directly to the shell IE or to source 32. With suchdirect connections, however, it has been found that the leads mayundesirably radiate high frequency energy from the resonator if carriedoutside of the resonator shell or ma give rise to parasitic oscillationsof a frequency different from that of the resonator. In an actual deviceof the type illustrated in Fig. 1 in which the operating frequencyranged from 2000 to 3000 megacycles parasitic oscillations at afrequency of about 750 megacycles were encountered. Such parasiticoscillations may be suppressed according to the invention in the mannerillustrated in Fig. 1. By an arrangement of the leads and the use ofby-passing condensers, small chokes and resistors the parasiticoscillation circuit comprising the leads I9 and 29 connecting to thediscs I2 and i3 is made to include damping resistors as shown in theschematic diagram Fig. 3. In Fig. 3, CI represents the capacitancebetween the two discs I2 and I3 which are connected to and support theelectrodes I and 3. The leads I9 and 29 form a transmission line betweenCI and C2. C2 represents a small by-passing capacitance made up of thebetween leads 2! and 22 and the member 24% separated by'the dielectricof the in sulating bushings 25 and 29. Thus the line comprising leads I9and 29 is terminated at the boundary of the resonator by the capacitanceCZ. In the actual 2000 to 3000-megacycle device referred to thiscapacitance was approximately one micromicrofarad. Resistors 29, and 30ohms each in the device mentioned) are connected across the terminatingcapacitance C2 but are isolated from the resonator oscillations (in the2000 to 3000-megacycle frequency range in the device mentioned) by thesmall choke coils 2? and 29 which in the actual device mentioned were 5turns each on a one-sixteenth inch inandrel. Thus the resistors areeffective in preventing the parasitic oscillations such as at the mentioned 759-megacycle frequency but are isolated from the resonatoroscillations at 2000 to 3000 megacycles. In order to have a low voltageacross the capacitance C2 at the resonator frequency, (though thevoltage is high between electrodes I and 8 and hence across thecapacitance CI) the line length between CI and C2 is made approximatelyone-quarter Wavelength long with respect to that frequency. This isaccomplished in the showing of Fig. l by building into the resonatorspace with the metallic block 2;. which shortens the line by effectivelymoving inwardly the cavity boundary at the position of the leads I9 and20. When the device is to cover a range of frequencies, as was theactual one referred to (where the range was 2000 to 3000 megacycles),and the length of the line cannot conveniently be varied, it isnecessary to compromise upon the length so that at some frequencies itdeparts from the quarter wavelength. In the actual device referred tothe length of the line varied between one-quarter and less than one-halfwavelength over the frequency range. Lengths of the order of one-halfwavelength should be avoided as then the voltage across C2 would tend tobe as high as that across CI. The line can be any odd number of quarterwavelengths long with the same relation between. the voltages across CIand C2. Similarly any number of half wavelengths should be avoided.

In the 2000 to 3000-megacycle device referred to, the arrangementdescribed was effective in removing parasitic oscillations atapproximately 750 megacycles without absorbing or permitting theradiation of appreciable power in the 2000 to 3000-megacycl range. Therequired electrical values of the elements C2, 2'1, 28, 29 and 30 willvary depending upon the operating frequency. The values are notcritical, however, these mentioned in connection with the references tothe 2000 to 3000-megacycle device tested are given only as illustrative.While it may be unusual it is possible that under som conditions theisolating choke coils 21 and '28 may not be required.

In review, the features of the invention which cooperate in preventingleads through a resonator from dissipating useful energy or causingparasitic oscillations are, briefly, lead lengths within the resonator,such, in terms of wavelength, that a low voltage termination at theresonator boundary may be had, termination of the leads at the boundaryto by-pass energy at the operating frequency, and termination of theleads external to the boundary to dissipate energy at a parasiticfrequency.

In Figs. 1 and 3 the connections are such that the electrodes 1 and 8are polarized through the leads l9 and to the same potential as theresonator shell Hi to which they are connected by lead 23. Theseelectrodes may be polarized at a potential different from that of theshell 13 as shown in Fig. 4. Fig. 4 is bounded by the broken line A andmay be substituted for the portion of Fig. 1 within the broken line A ofthat figure. Here a by-pass condenser 33 is interposed between the lead'23 and the shell l6 and lead 23 is connected to the potential sourcethrough lead 34 as may be desired. Should it be desired to polarize theelectrodes I and 8 to different potentials th connection betweenresistors 29 and 39 may be opened and each resistor connected to theshell through separate by-pass condensers each like condenser 33 andeach resistor connected to a. desired point on the potential source by aseparate lead like lead 34.

In connection with Fig. 1 it was explained that the member 24 is used toshorten the length of the leads 1!] and 20 within the resonator and thatit also served in conjunction with leads 2| and 22 to form the by-passcapacitance designated C2 in Fig. 3. In case the wavelength requirementof leads I9 and 20 does not require that they be shortened by buildinginto the resonator space as by member 24 in Fig. 1 it will be necessaryto provide otherwise the by-pass capacitance C2. One method of doingthis is shown in Figs. 5 and 6 where a metallic member is connected tothe sh'ell 16 external to it. The leads 2! and 22 pass through member 35and with it and the dielectrio of the insulating bushings 25 and 26 forma by-pass capacitance in the same manner as with member 24 in Fig. 1.Fig. 5 may be substituted in Fig. 1 for the portion of Fig. 1 bounded bythe broken line A.

It is obvious that, if rather than the necessity for shortening thelength of conductors l9 and 20 by building into th'e resonator space asshown in Figs. 1 and 4 it is necessary to increase the length of thconductors, the resonator space may be built out by means of a bulgeoutward of member I6 at the position of the conductors I9 and 23.

Fig. 7 shows the application of the invention when the electron tube ofFig. 1 is a two-gap tube rather than a three-gap tube. It will be notedin Fig. 7 that between electrodes 6 and 9 there is one electrode 31 andtwo interelectrode gaps rather than two electrodes (l and 3) and threeinterelectrode gaps as shown in Figs. 1, 4 and 5. Fig. 7 may besubstituted in Fig. 1 for the portion of Fig. 1 bounded by the brokenline A. The operation with Fig. '7 substituted in Fig. 1 is the same asexplained in connection with Fig. 1 except that the second gap of Fig. 1does not exist and may be considered as eliminated by the extension ofelectrodes 1 and 8 toward each other to close it. In operation with Fig.'7 the electron velocities are varied in the gap between electrodes 6and 31, they are grouped and the electron stream becomes density variedwithin electrode 31 and energy is transferred from the electron streamto the high frequency electric field of the resonator in the gap betweenelectrodes 31 and 9. As regards the present invention, the Fig. 7modification of Fig. 1 alters the situation in that only one lead 38 isrequired within the resonator space rath'er than the two I9 and 23. Asindicated in Fig. 7 the only effect of this is to eliminate one leadsuch as I 9 and its associated through lead and bushing, choke coil andresistor. That is, it may be seen that in effect it eliminates from Fig.1 lead l9, lead 2| and bushing 25, choke coil 21 and resistor 29.correspondingly, it eliminates choke coil 2'? and resistor 29 from Fig.3 leaving the schematic diagram as shown in Fig. 8. The diagram of Fig.8 applies to Fig. '7 just as the diagram of Fig. 3 applies to Fig. 1. InFig. 8, C3 represents the capacity of electrode 31 to electrodes E and 3and C4 represents the by-pass capacitance formed by the conductor 39 thedielectric of bushing 40 and member M. It is obvious that thearrangement of Figs. 7 and 8 functions to prevent parasitic oscillationand the dissipation of energy at the operating frequency in the samemanner as the arrangement of Fig. 1. It may be said that Fig. 3 shows abalanced line arrangement While Fig. 8 shows an unbalanced line.

It is obvious that the modifications of Fig. 1 to provide a difierentbias on the electrodes in the resonator and to provide a by-passcapacity C2 without shortening the leads within th resonator shown inFigs. 4 and 5, respectively, may also apply to the arrangement shown inFig. 7.

The circuit shown to illustrate the invention is that of an oscillator.However, it should be understood that the utility of the invention isnot so limited and that it may be used with amplifiers or any otherdevice wherein it is desired to carry conductors through the space of anelectrical resonator and it is necessary to prevent parasiticoscillations which may be produced thereby. Further, it may beapplicable to any high frequency circuit where a similar conditionexists whether or not a space resonator is involved. For instance, theremay be involved the carrying of leads through a non-resonant wave guideor a shielded compartment or even through a region of high frequencyinfluence without a definite boundary.

What is claimed is:

1. A high frequency system comprising means for producing in a region ahigh frequency electric field, at least one electrical conductor capableof acting as a high frequency transmission line in conjunction withanother part of the system serving as a return conductor and extendinginto the said region from a region in which the high frequency field issubstantially less intense than in the first said region, the length ofthe said electrical conductor within the first said region beingsubstantially a quarter wavelength of the high frequency field, acapacitance connected between the said conductors at the boundary of thefirst said region, and an inductance and a resistance connected inseries between the said conductors in the second said region.

2. A high frequency system comprising means for producing in a region ahigh frequency electric field, at least one electrical conductor capableof acting as a high frequency transmission line in conjunction withanother part of the system serving as a return conductor and extendinginto the said region from a region in which the high frequency field issubstantially less intense than t i 1 ml in the first said region, thelength of the said electrical conductor within the first said regionbeing substantially a quarter wavelength of the high frequency field, acapacitance connected between the said conductors at the boundary of thefirst said region, and a resistance connected between the saidconductors in the second said region.

3. A high frequency system comprising means for producing in a region ahigh frequency electric field, at least one electrical conductor capableof acting as a high frequency transmission line in conjunction withanother part of the system serving as a return conductor and extendinginto the said region from a region in which the high frequency field issubstantially less intense than in the first said region, the length ofthe said electrical conductor within the first said region beingsubstantially an odd number of quarter wave-lengths of the highfrequency field, a capacitance connected between the said conductors, atthe boundary of the first said region, and an inductance and aresistance connected in series between the said conductors in the secondsaid region.

4. A high frequency system comprising means for producing in a region ahigh frequency electric field, at least one electrical conductor capableof acting as a high frequency transmission line in conjunction withanother part of the system servin as a return conductor and extendinginto the said region from a region in which the high frequency field issubstantially less intense than in the first said region, the length ofthe said electrical conductor within the first said region beingsubstantially an odd number of quarter wavelengths of the high frequencyfield, a capacitance connected between the said conductors at theboundary of the first said region, and a resistance connected betweenthe said conductors in the second said region.

5. In combination, a hollow electrical resonator, lead extending frompoints within the resonator to points external to it, means fordetermining the lengths of the leads within the resonator appropriatelywith respect to the frequency to which the resonator is tuned, a shuntpath of low impedance at the frequency of the resonator between theleads in the region of the resonator boundary, and a resistive loadconnected between the leads external to the resonator.

6. A high frequency device comprising a hollow electrical resonator, anelectron tube at least partially included within the space of theresonator, electrodes of the said tube within the space of theresonator, electrical conductors connected to the said electrodes andextending across the space of the resonator to and beyond the boundarythereof, means for determining the distance required to be traversed bythe conductors between the electrodes and the resonator boundary,capacitance means interconnecting the conductors where they leave theresonator space, and inductance and resistance means interconnecting theconductors outside of the resonator space.

'7. A device according to claim 6 in which the distance required to betraversed by the conductors between the electrodes and the resonatorboundary is made substantially one-quarter wave length at the resonantfrequency of the resonator.

8. A device according to claim 6 in which the distance required to betraversed by the conductors between the electrodes and the resonatorboundary is made substantially an 'odd number of quarter wavelengths atthe resonant frequency of the resonator.

9. A high frequency device comprising a hollow electrical resonatorbounded by a shell of electrically conducting material, an electron tubeat least partially included within the space of the resonator, anelectrode of the said tube within the space of the resonator, anelectrical conductor connected to the said electrode and extendingacross the space of the resonator to and beyond the boundary thereof,means for determining the distance required to be traversed by theconductor between the electrode and the resonator boundary, capacitancemeans interconnecting the conductor and the resonator shell where theconductor leaves the resonator space and an electrical connectionincluding inductance and resistance interconnecting the conductor andthe resonator shell outside of the resonator space.

10. A device according to claim 9 in which the distance required to betraversed by the conductor between the electrode and the resonatorboundary is made substantially one-quarter wavelength at the resonantfrequency of the resonator.

11. A device according to claim 9 in which the distance required to betraversed by the conductor between the electrode and the resonatorboundary is made substantially an odd number of quarter wavelengths atthe resonant frequency of the resonator.

12. A high frequency device comprising a hollow electrical resonatorbounded by a shell of electrically conducting material, an electron tubeat least partially included within the space of the resonator, anelectrode of the said tube within the space of the resonator, anelectrical conductor connected to the said electrode and extendingacross the space of the resonator to and beyond the boundary thereof,means for determining the distance required to be traversed by theconductor between the electrode and the resonator boundary, capacitancemeans interconnecting the conductor and the resonator shell where theconductor leaves the resonator space and an electrical connectionincluding resistance interconnecting the conductor and the resonatorshell outside of the resonator space.

13. A device according to claim 12 in which the distance required to betraversed by the conductor between the electrode and the resonatorboundary is made substantially one-quarter wavelength at the resonantfrequency of the resonator.

141. A device according to claim 12 in which the distance required to betraversed by the conductor between the electrode and the resonatorboundary is made substantially an odd number of quarter wavelengths atthe resonant frequency of the resonator.

15. A device according to claim 12 in which the distance required to betraversed by the conductor between the electrode and the resonatorboundary is made substantially less than one-half wavelength at theresonant frequency of the resonator.

16. A high frequency device comprising a hollow electrical resonatorhaving a shell of conducting material, an electrical conductor extendingfrom a position within the space of the resonator to the boundary of theresonator, means for determining the distance necessary to be traversedby the conductor within the space of the resonator comprising aconducting member built into the resonator space at the position of theconductor and connected to the shell at that position to effectivelymove the shell inwardly to reduce the space in the resonator and thedistance required to be traversed by the conductor in reaching theboundary of the resonator.

17. A high frequency oscillator system comprising a hollow resonatorserving as a resonant circuit whereby the oscillator is made operative-at a given high frequency, an electron discharge device in operativerelation to the said resonator, at least one electrical conductorcapable of acting as a high frequency transmission line in conjunctionwith another part of the system serving as a return conductor andextending from without the resonator through the space within theresonator to connect with an electrode of the electron discharge device,the length of the said electrical conductor within the space of theresonator being substantially an odd number of quarter wavelengths ofthe said given high frequency, a capacitance connected between the saidconductors at the boundary of the resonator and a resistance connectedbetween the said conductors external to the resonator.

18. A high frequency system comprising a hollow resonator serving as aresonator circuit whereby the system is made operative at a given highfrequency, an electron discharge device in operative relation to thesaid resonator, at least one electrical conductor capable of acting as ahigh frequency transmission line in conjunction with another part of thesystem serving as a return conductor and extending from without theresonator through the space within the resonator to connect with anelectrode of the electron discharge device, the length of the saidelectrical conductor within the space of the resonator beingsubstantially an odd number of quarter wavelengths of the said givenhigh frequency, a capacitance connected between the said conductors atthe boundary of the resonator and a resistance connected between thesaid conductors external to the resonator.

19. A high frequency system comprisin a hollow resonator having a shellof conducting material and serving as a resonant circuit whereby thesystem is made operative at a given high frequency, at least oneelectrical conductor entering and passing through the space of theresonator, and means for making the length of the said conductor withinthe space a desired value with respect to the wavelength correspondingto the said high frequency comprising a structure of conducting materialdesigned with respect to the said desired conductor length and theresonator dimensions placed within the shell of the cavity and incontact therewith in the region where the conductor enters, in effectthickening the shell of the cavity in that region.

JAMES W. McRAE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,296,678 Linder Sept. 22, 19422,281,717 Samuel May 5, 1942

